soft frames offer decent protection from normal handling (i've shipped bikes a whole bunch), but what they are not designed for is:
1 - getting run over by a vehicle. a hard case may prevent damage here.
2 - falling out of a plane (yes, i've seen a bike miss the conveyer belt and fall from a 747's cargo bay), but a hard case is not likely to help here because....
3 - TSA rummaging through your stuff and improperly re-packing it.

I like the "BikND" Jetpack I ended up getting, since it's a soft-shell but with a frame, and inflated donuts on both sides to protect the wheels. Not sure about #1, but I'm pretty confident with it around #2 & #3. There's a video of it being dropped from a 2nd story and the bike is fine.

And in the 4-5 times I've used it, TSA has opened it up but they don't need to unpack anything since it's pretty open inside and they can get in to it from both sides.

The downside of course is that it's bigger, hard to get away with the "it's not a bike" thing. Even if you convince them it's "art" it's still obviously oversized. At least it comes in under 50 lbs when packed, so you don't get dinged twice.

The trick is to fly out of smaller airports - 3 out of 4 times at Mobile Regional they just check it like normal baggage, no questions asked. At SFO, ATL, SJC it's a little trickier. Sometimes you can talk them down a bit from the bigger fees though.

I'm just amazed every time I travel with the carbon-everything bike and it comes out on the other end unscathed.